To show that the just life is better, Socrates first takes up the question of what justice is (Republic II.368c).

This is what the reader might expect, given his practice in the early dialogues in which he searches for a definition, but now, in the Republic, Socrates proceeds in an unexpected way. He does not search simply for what justice is. Instead, he searches for what justice is in a city and what it is in an individual. He says that he will search for what justice is in a city, because it is "bigger," and so presumably more straightforward to consider, and then, with the nature of justice in the city understood, he will search for what justice is in a human being.

This strategy can seem puzzling, but the Gorgias provides some explanation. Socrates says that when someone "does what is fitting (προσήκοντα) as regards men, his actions will be just (δίκαι᾽)..." (Gorgias 507a). A few lines earlier he says that "when a certain order, the proper (οἰκεῖος) one for each thing, is present, a thing is good" (Gorgias 506e).

Remember also that Thrasymachus demanded that Socrates not explain what justice is by supplying a synonym. A synonym is not informative. He wants Socrates to say what justice is in terms of the underlying facts. "[D]o you yourself answer, Socrates, and tell us what you say the just is. And don't you be telling me that it is that which ought to be, or the beneficial or the profitable or the gainful or the advantageous, but express clearly and precisely whatever you say. For I won't take from you any such drivel as that" (Republic I.336d).

These passages help explain the strategy Socrates pursues in the Republic. The suggestion is that the correct answer to the "What is justice?" question is the relatively uninformative one that justice is what is appropriate with respect to human beings. In the Republic, unlike in the early dialogues, the pursuit of a definition is no longer what drives the discussion. The search is for justice is in a city and in an individual. Socrates begins with the search for justice in the city. This is a search for the proper organization of human beings into cities.

What Justice is in the City

Human beings organize themselves in cities to make their lives better. Cities provide the benefits of group living, but there are appropriate and inappropriate ways that human beings can organize themselves into a city. The appropriate organizations constitute justice in the city. These organizations function in a certain way.

Socrates says these organizations function so that the city has three parts. It has a guardian, or ruling, class. The rulers are lovers of wisdom (Republic V.473d). They introduce the rules for the production and distribution of goods and for the behavior of individuals in the city more generally. The city also has an auxiliary class. Its job is to enforce the rules. Finally, the city has a working class to produce the services and material goods. The rulers must rule, the auxiliaries must enforce the rules, and the workers must produce the services and material goods.

"I think our city, if it has been rightly (ὀρθῶς) founded, is completely good (τελέως ἀγαθὴν). Clearly, then, it will be wise (σοφή), brave (ἀνδρεία), moderate (σώφρων), and just (δικαία). So if we find any of these qualities in it, the remainder will be that which we have not found" (Republic IV.427e).

Socrates says that this city is wise because the rulers are wise, brave because the auxiliaries are brave, moderate because everyone is controlled, and just because each of three parts does its own job.

Now that they have found justice in the city, they apply their results to justice in an individual human being.

To apply their results to justice in the individual human being, they appeal to a principle about the use of words to call many things by the same name. The idea is that when someone predicates justice of many things ("This city is just." "This human being is just.") he is saying the same thing each of the many things he calls by the same name.

"If you call a thing by the same name whether it is big or little, it is like in the way in which it is called the same or like. Then a just man too will not differ at all from a just city in respect of the very form of justice, but will be like it (καὶ δίκαιος ἄρα ἀνὴρ δικαίας πόλεως κατ᾽ αὐτὸ τὸ τῆς δικαιοσύνης εἶδος οὐδὲν διοίσει, ἀλλ᾽ ὅμοιος ἔσται). But now the city was thought to be just because three natural kinds existing in it performed each its own function, and again it was temperate, brave, and wise because of certain other affections and habits of these three kinds. Then, my friend, we shall thus expect the individual also to have these same forms in his soul, and by reason of identical affections of these with those in the city to receive properly the same appellations" (Republic IV.435a-c).

Given their understanding of justice in the city as the proper organization of the parts of the city, and this principle about the use of words to say one thing about many objects, it seems to follow that the soul has parts. Socrates admits that this question of parts is difficult, but he appeals to a principle about opposites to show that the human soul is tripartite.

"The matter begins to be difficult when you ask whether we do all these things with the same thing or whether there are three things and we do one thing with one and one with another—learn (θυμούμεθα) with one part of ourselves, feel anger (θυμούμεθα) with another, and with yet a third desire (ἐπιθυμοῦμεν) the pleasures of nutrition and generation and their kind, or whether it is with the entire soul (ἢ ὅλῃ τῇ ψυχῇ) that we function in each case when we once begin. That is what is really hard to determine properly" (Republic IV.436a-b).

The Tripartite Theory of the Soul

In the Republic, Plato has Socrates introduces a new understanding of the soul.

The soul now has three parts: "reason" (τὸ λογιστικὸν) "spirit" (τὸ θυμοειδές), and "appetite (τὸ ἐπιθυμητικόν)." All parts of the soul have desires, but desire in appetitive and spirited parts is not a matter of belief about what is good and what is bad. The desires in these parts arise independently of any beliefs about what is good or bad.

In the Tripartite Theory of the Soul, Plato abandons the Socratic intellectualist theory Socrates advocates in the Protagoras.

The Argument from Opposites

Sometimes a person is thirsty but refuses to drink. How is this possible? According to the Tripartite Theory, there are two desires in play. One stems from appetite. It arises naturally in reaction to events in the body. In the absence of a desire from reason, this appetitive desire moves the person to drink. Reason, however, in the passage (below), has the belief that in the circumstance drinking is not good. If reason rules, the connection between the appetitive desire and action is interrupted. Reason overrides appetite.

"Is there something in the soul of those who are thirsty but refuse to drink, something bidding them to drink and something different, something forbidding them, that overrides the thing that bids them to drink? And doesn't the thing that forbids in such cases come into play, if it comes into play, as a result of calculation (λογισμοῦ), while what drives and drags them to drink is a result of feelings (παθημάτων) and diseases? Hence isn't it right for us to claim that they are two, and different from one another? We'll call the part of the soul with which it reasons the λογιστικὸν and that with which it lusts, hungers, thirsts, and feels other appetites, the irrational (ἀλόγιστόν) and appetitive part (ἐπιθυμητικόν), companion of indulgences and pleasures" (Republic IV.439c-d).

Socrates argues for this understanding in terms of a principle about opposite motions. "It is obvious that the same thing will never do or suffer opposites in the same respect in relation to the same thing and at the same time. So that if ever we find this happening we shall know that it was not the same thing but a plurality" (Republic IV.436b-436c).

The idea is that desire and aversion are opposite motions of the soul. Desire is a motion toward, and aversion is a motion away. If a person is thirsty, he has a motivation to drink. If he thinks that drinking is not in his best interest, he also has a motivation not to drink. If this desire and aversion are opposite motions, then given the principle about opposite motions, it follows that this desire and aversion are motions of different things in the person. Since human beings are psychological beings, and thus do whatever they do because of states and processes in their "soul" (ψυχή), Socrates concludes that the human psychology has at least two parts. The appetitive part of the soul has the desire, and the part of the soul that reasons has the aversion.

In addition to the part with reason (the λογιστικὸν) and the part with appetites (the ἐπιθυμητικόν), Socrates argues for a part with spirit (θυμοειδές).

"Of the spirit (θυμοῦ), that with which we feel anger (θυμούμεθα), is it a third, or would it be the same as these [we have distinguished]" (Republic IV.439e)? "So in the soul, there is the spirited part (θυμοειδές), which is the helper of reason by nature (τῷ λογιστικῷ φύσει) unless it is corrupted by bad nurture" (Republic IV.441a)?

Socrates says that some part of the soul conflicts with appetite in the case of Leontius (Republic IV.439e-440a), that children have spirit, but that "as for reason (λογισμοῦ), some of them, to my thinking, never participate in it, and the majority quite late" (Republic IV.441a-b). The argument seems to be that examples of the conflict in the case of Leontius are seen in children and in animals (Republic IV.441b), that animals lack reason all together, and that reason does not play a controlling role in the actions of children.

The Proper Organization of the Parts of the Soul

Given the Tripartite Theory of the Soul, there are different possible organizations among the parts of the soul. The proper organization is the one in which reason rules, spirit is reason's ally, and appetite is suppressed. When the parts are so organized, they are in "harmony." Since reason knows what is good, a human being whose soul is in "harmony" acts for the sake of the good.

"It is appropriate for the reasoning part to rule, since it is wise and exercises foresight on behalf of the whole soul, and for the spirited part to obey and be its ally (οὐκοῦν τῷ μὲν λογιστικῷ ἄρχειν προσήκει, σοφῷ ὄντι καὶ ἔχοντι τὴν ὑπὲρ ἁπάσης τῆς ψυχῆς προμήθειαν, τῷ δὲ θυμοειδεῖ ὑπηκόῳ εἶναι καὶ συμμάχῳ τούτου)" (Republic IV.441e4-6).

What Justice is in the Individual

Socrates concludes that justice in the individual is the proper organization of the parts of the "soul" (ψυχή). The proper organization of these parts is one in which the three parts are in "harmony" (ἁρμονία). In this organization, "reason" (τὸ λογιστικὸν) rules and takes "spirit" (τὸ θυμοειδές) as its ally against "appetite" (τὸ ἐπιθυμητικόν).

"It is fitting for the reasoning part to rule (ἄρχειν), it being wise and exercising foresight on behalf of the whole soul, and for the spirited part to obey it and be its ally" (Republic IV.441e)."These two parts... will exercise authority over the appetitive part which is the largest part and is insatiable for possessions. They will watch over it to see that it is not filled with so-called pleasures of the body, and by becoming enlarged and strong thereby no longer does its own job but attempts to enslave and rule over those over whom it is not fitted to rule, and so upsets everyone's whole life" (Republic IV.442a-b).

This organization of the three parts of the soul ("reason" (τὸ λογιστικὸν), "spirit" (τὸ θυμοειδές), and "appetite" (τὸ ἐπιθυμητικόν)) is justice in a human being.

"[Justice] does not lie in a man's external actions, but in the way he acts within himself.... He does not allow each part of himself to perform the work of another, or the parts of his soul (τὰ ἐν τῇ ψυχῇ γένη) to meddle with one another. He regulates well what is really his own and rules himself. He puts himself in order, ... and harmonizes the three parts of himself... He thinks that the just and beautiful action, which he names as such, to be that which preserves this state (ἕξιν) and indeed helps achieve it, wisdom to be the knowledge which oversees this action; and believing and naming the unjust action to be that which ever tends to, an unjust action to be that which always destroys it, and ignorance the belief which oversees that" (Republic IV.443d-444a).

A solution is to a puzzle about "wisdom and truth and the best state of [the] soul"

Socrates, in the early dialogues, suggests that there is a certain competency involved in living a good life, that this competency is a state of the soul, and that the person who has the traditional virtues is the one who has the competency involved in living a good life. The person who acts in terms of these virtues is arranging things so that he his living a good life.

In the Republic, Socrates makes it clearer how the traditional virtues are connected to the competency involved in living a good life and is "happy" (εὐδαίμων).

Socrates explains that the state of the soul is justice, that justice in the soul is a matter of the three parts of the soul each doing its own job. The part with reason leads. It is not confused about what is good and what is bad. In this way, the person is wise because the part with reason exercises "forethought in behalf of the entire soul" and hence arranges things so that he lives a good life. He courageous and brave because the spirited part of his soul holds to the declarations of reason against the desires of appetitive part. He is temperate because the appetitive part is controlled.

  "Then, wouldn't these two parts also do the finest job of guarding the whole soul and the body (ἁπάσης τῆς ψυχῆς τε καὶ τοῦ σώματος) against external enemies--reason by planning, spirit by fighting, following its leader, and carrying out the leader's decision through its courage?
  Yes, Socrates, that is true.
  And it is because of the spirited part (τὸ θυμοειδὲς), I suppose, that we call a single individual courageous (ἀνδρεῖον), namely, when it preserves through pains and pleasures the declarations of reason about what is to be feared and what isn't.
  That is right.
  And we'll call him wise (σοφὸν) because of that small part of himself that rules in him and makes those declarations and has within it the knowledge (ἐπιστήμην) of what is advantageous for each part and for the whole soul, which is the community of all three parts.   And isn't he temperate (σώφρονα) because of the friendly and harmonious relations between the same parts, namely when the ruler and the ruled believe in common that the rational part should rule and don't engage in civil war against it?
  Temperance is surely nothing other than that, both in the city and in the individual" (Republic IV.442b-d).